Hydrodynamic fluid bearings are being increasingly used, for example, in rapidly rotating hard disk drives instead of roller bearings, due among other things to their low-noise level and shock resistance. Hydrodynamic fluid bearings, also called fluid dynamic or hydrodynamic bearings (or journal bearings), consist of at least one stationary and one rotating component whose active surfaces are separated from each other by a space filled with a lubricant, preferably oil.
To build up hydrodynamic pressure in the bearing space, at least one of the bearing surfaces has a groove or trench-like pattern formed in it. Due to the relative rotary movement of the component surfaces affecting each other, a kind of pump effect is created allowing a uniformly thick and homogeneous film of lubricant to be formed which is stabilized by appropriate zones of hydrodynamic pressure.
By means of such hydrodynamic bearings, for whose manufacture the invention finds application, the extremely precise rotation of a moving rotor, driven by an electric motor and with a high speed compared to a stationary stator, can be achieved.
In one embodiment, such a fluid bearing consists of a shaft with a thrust plate, a sleeve surrounding the shaft, a counter plate built up as a counter bearing attached to the sleeve. Depending on whether the shaft is stationary or moves, at least one of the surfaces of the thrust and/or counter plate and of the shaft or the sleeve has an appropriate grooved or trench-like pattern which can be formed by the electrode used in the present invention through electrochemical machining.
In the prior art it was previously known to form the groove pattern by mechanically removing material from the precision pre-machined surface of the bearing or by plastic deformation of the surface. In both methods, the material along the groove or other feature is removed and displaced using a mechanical process. The disadvantage of this mechanical machining process is that the removal or displacement of the material results in small eruptions of material along the edges of the feature which finally have to be removed in a relatively costly and difficult finishing process.
Here it should be noted that the grooves in hydrodynamic fluid bearings are very small and filigree and have to be formed with extreme precision, in the range of 1 to 2 μm.
Alongside the viscosity of the lubricant and the thickness of the lubricating film, the formation of these grooves are significant for the stiffness of the hydrodynamic bearing. Any variations in dimensions due to a lack of precision in forming the groove pattern result in pressure differences or pressure deviations and directly impair the precise running of the motor.
To achieve the highest possible bearing stiffness, efforts are made to minimize the thickness of the lubricating film. This leads to the requirement that the bearing space, that is the distance between the opposing bearing surfaces, be as small as possible. It is therefore absolutely essential that the grooves are formed with the utmost precision and with the lowest possible tolerances and in particular without the pile up of any material along the edges.
As well as describing the mechanical machining of the surfaces of hydrodynamic fluid bearings, DE 199 50 463 A1 also describes an alternative kind of machining in which the surface of a fluid bearing is coated and the desired features are formed in the coating by a beam of energy, in particular a laser beam.
An electrochemical method of forming grooves on the surface of a hydrodynamic fluid bearing is described in U.S. Pat. No. 6,267,869 B1. This document is based on a prior art in which the structured surface of a hydrodynamic bearing is formed by electrochemical etching whereby the electrode and the bearing surface are immersed in a salt solution, a difference in potential is created and the movement of the electrode defines the desired features and, in the way of a paintbrush, forms the surface. Although this process avoids the problem of material pile up along the edges of the grooves, it is still very slow and costly.
Consequently, U.S. Pat. No. 6,267,869 B1 suggests the use of a wide-coverage electrode that essentially covers the entire surface of the fluid bearing and which has appropriate grooves formed in it which mirror the grooves to be formed in the bearing surface. The workpiece and electrode are placed in a container filled with electrolyte and set at a precisely defined distance to each other.
By setting up a potential difference whereby the workpiece is connected as an anode and the tool electrode as a cathode, an electrochemical etching process is created.
While the electrolyte flows between the two electrodes, atom by atom is dissolved from the surface of the workpiece in accordance with Faraday's Law. Here, the weight of the dissolved metal is equivalent to the amount of electricity exchanged between cathode and anode.
With this method, described in the industry by such companies as “Extrude Hone”, Irwin, Pa., USA or “Loadpoint Ltd.” from Swindon, Wiltshire, UK, as ECM (Electro Chemical Machining) or EMM (Electro Micro Machining), the complete set of grooves on the surface of a shaft, sleeve or counter plate etc. can be formed in one processing step by fashioning the desired groove pattern, which is to be formed on the bearing surface, on the surface of the tool electrode.
In U.S. Pat. No. 6,267,869 B1 it is described that the tool electrode can be formed by mechanical machining and although the problem of material pile up can again occur here, due to the repeated use of the electrode the cost of post-machining can be relativized. After mechanical machining, the tool electrode is embedded in an insulating plastic substance, for example in a two-component resin, to ensure that the transport of electrical charges only takes place in the area of the “active”, that is the free electrode surface, otherwise the surrounding surfaces could be undesirably etched due to parasitic charge exchange.
Reference is made to U.S. Pat. No. 6,267,869 B1 particularly to the extent that it applies to the use of the electrode in forming the bearing surfaces.
In the method described above for manufacturing a tool electrode to form grooves on a hydrodynamic fluid bearing, the problem still exists that the manufacture of such electrodes is time consuming and cost intensive, particularly when microfeatures are to be formed with high precision and the surface of the electrode is curved. The risk of damaging such electrodes is very high even with careful handling. The structured electrode surface is made of e.g. copper, brass, aluminum or nickel which are relatively soft and thus easy to damage mechanically. Due to the high electrical current the electrolyte heats up to 80° C. to 100° C. which can place considerable thermal strain on the electrode, and due to the different thermal expansion coefficients of the metallic electrode material and the insulating plastic, a loosening of the bond and final breaking or splitting of the insulation occur.
With an electrode of the said art, as a rule no more than about 50,000 surfaces can be machined which corresponds to less than one day's production. According to its nature, each electrode is unique and due to the mechanical machining of the electrode surface, the duplication of its surface structure is only possible within specific work tolerances.
Consequently, it is the object of the invention to provide a method for manufacturing an electrode of the art described above as well as an appropriate electrode, which overcomes the problems described and particularly by which an electrode can be manufactured with high precision and reproducibility at a low cost.